Lithium silicate materials

ABSTRACT

Lithium silicate materials are described which can be easily processed by machining to dental products without undue wear of the tools and which subsequently can be converted into lithium silicate products showing high strength.

This application is continuation of U.S. application Ser. No.15/255,443, filed on Sep. 2, 2016, which is a continuation of U.S.application Ser. No. 14/567,173, filed on Dec. 11, 2014, now U.S. Pat.No. 9,434,639, issued on Sep. 6, 2016, which is a continuation of U.S.application Ser. No. 13/940,362, filed on Jul. 7, 2013, now U.S. Pat.No. 8,940,651, issued on Jan. 27, 2015, which is a continuation of U.S.application Ser. No. 13/175,370, filed on Jul. 1, 2011, now U.S. Pat.No. 8,546,280, issued on Oct. 1, 2013, which is a continuation of U.S.application Ser. No. 12/944,578, filed on Nov. 11, 2010, now U.S. Pat.No. 7,993,137, issued on Aug. 9, 2011, which is a continuation of U.S.application Ser. No. 12/253,437, filed on Oct. 17, 2008, now U.S. Pat.No. 7,871,948, issued on Jan. 18, 2011, which is a divisional of U.S.application Ser. No. 11/348,053, filed on Feb. 6, 2006, now U.S. Pat.No. 7,452,836, issued on Nov. 18, 2008, which claims priority toEuropean Patent Application No. EP 05002588.1, filed on Feb. 8, 2005,and to German Patent Application No. 10 2005 028 637.2, filed on Jun.20, 2005, each of which are herein incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The invention primarily relates to lithium silicate materials which canbe easily shaped by machining and subsequently converted into shapedproducts with high strength.

BACKGROUND OF THE INVENTION

There is an increasing demand for materials which can be processed intodental restorative products, such as crowns, inlays and bridges, bymeans of computer controlled milling machines. Such CAD/CAM methods arevery attractive as they allow to provide the patient quickly with thedesired restoration. A so-called chair-side treatment is thus possiblefor the dentist.

However, materials suitable for processing via computer aideddesign/computer aided machining (CAD/CAM) methods have to meet a veryspecific profile of properties.

First of all, they need to have in the finally prepared restorationappealing optical properties, such as translucence and shade, whichimitate the appearance of the natural teeth. They further need to showhigh strength and chemical durability so that they can take over thefunction of the natural tooth material and maintain these propertiesover a sufficient period of time while being permanently in contact withfluids in the oral cavity which can even be aggressive, such as acidicin nature.

Secondly and very importantly, it should be possible to machine them inan easy manner into the desired shape without undue wear of the toolsand within short times. This property requires a relatively low strengthof the material and is therefore in contrast to the desired propertiesmentioned above for the final restoration.

The difficulty of combining the properties of low strength in the stageof the material to be processed and a high strength of the finalrestoration is reflected by the known materials for a CAD/CAM processingwhich are in particular with respect to an easy machinabilityunsatisfactory.

DE-A-197 50 794 discloses lithium di silicate glass ceramics which areprimarily intended to be shaped to the desired geometry by ahot-pressing process wherein the molten material is pressed in theviscous state. It is also possible for these materials to be shaped bycomputer aided milling processes. However, it has been shown that themachining of these materials results in a very high wear of the toolsand very long processing times. These disadvantages are caused by thehigh strength and toughness primarily imparted to the materials by thelithium disilicate crystalline phase. Moreover, it has been shown thatthe machined restorations show only a poor edge strength. The term “edgestrength” refers to the strength of parts of the restoration having onlya small thickness in the range of few 1/10 mm.

Further approaches of achieving easy machinability together with a highstrength of the final restoration have also been made. EP-B-774 993 andEP-B-817 597 describe ceramic materials on the basis of Al₂O₃ or ZrO₂which are machined in an unsintered state which is also referred to as“green state”. Subsequently, the green bodys are sintered to increasethe strength. However, these ceramic materials suffer from a drasticalshrinkage of up to 50% by volume (or up to 30% as linear shrinkage)during the final sintering step. This leads to difficulties in preparingthe restorations with exactly the dimensions as desired. The substantialshrinkage represents a particular problem if complicated restorationsare manufactured, such as a multi-span bridge.

From S. D. Stookey: “Chemical Machining of Photosensitive Glass”, Ind.Eng. Chem., 45, 115-118 (1993) and S. D. Stookey: “PhotosensitivelyOpacifiable Glass” U.S. Pat. No. 2,684,911 (1954) it is also known thatin lithium silicate glass ceramics a metastable phase can be formed atfirst. For example in photosensitive glass ceramics (Fotoform®,FotoCeram®) Ag-particles are formed using UV-light. These Ag-particlesserve as crystallization agent in a lithium metasilicate phase. Theareas which were exposed to light are in a subsequent step washed out bydiluted HF. This procedure is possible since the solubility of thelithium metasilicate phase in HF is much higher than the solubility ofthe parent glass. The glass portion remaining after said solubilizingprocess (Fotoform®) can be transferred into a lithium disilicate glassceramic (FotoCeram®) by an additional heat treatment.

Also investigations of Borom, e.g. M.-P. Borom, A. M. Turkalo, R. H.Doremus: “Strength and Microstructure in Lithium DisilicateGlass-Ceramics”, J. Am. Ceram. Soc., 58, No. 9-10, 385-391 (1975) andM.-P. Borom, A. M. Turkalo, R. H. Doremus: “Verfahren zum Herstellen vonGlaskeramiken” DE-A-24 51 121 (1974), show that a lithium disilicateglass ceramic can in the first instance crystallize in varying amountsas metastable lithium metasilicate phase. However, there also existcompositions which crystallize in the form of the disilicate phase fromthe beginning and the metasilicate phase is not present at all. Asystematic investigation of this effect has not become known. From theinvestigations of Borom it is also known that the glass ceramic whichcontains lithium metasilicate as the main phase has a reduced strengthcompared to the one of a glass ceramic which only contains a lithiumdisilicate phase.

It has further been found out that the presence of ZnO in lithiumsilicate glass ceramics of the prior art is undesirable especially whenhighly translucent dental restorations are to be produced. Under suchcircumstances, the strong opalescent effect caused by ZnO is apparentand results in unacceptable optical properties for a restoration whichis to imitate the natural tooth material.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to eliminate thesedisadvantages and in particular to provide a material which can beeasily shaped by computer-aided milling and trimming processes and cansubsequently be converted into a high-strength dental product which alsodisplays a high chemical durability and excellent optical properties andexhibits a drastically reduced shrinkage during said final conversion,and achieves all these properties without the need for ZnO as acomponent.

This object is achieved by the lithium silicate glass ceramic accordingto the invention.

DETAILED DESCRIPTION OF THE INVENTION

It has surprisingly been shown that by using a starting glass of a veryspecific composition and a specific process it is possible to provide inparticular a glass ceramic which has metastable lithium metasilicate(Li₂SiO₃) as main crystalline phase rather than lithium disilicate(Li₂Si₂O₅). This lithium metasilicate glass ceramic has a low strengthand toughness and hence can be easily machined into the shape of evencomplicated dental restorations, but can after such machining beconverted by a heat treatment into a lithium disilicate glass ceramicproduct with outstanding mechanical properties, excellent opticalproperties, in particular a strongly reduced opalescence, and very goodchemical stability thereby undergoing only a very limited shrinkage.

The lithium silicate glass ceramic according to the invention comprisesthe following components:

Component wt. -% SiO₂ 64.0-75.0, in particular 64.0-73.0 Li₂O 13.0-17.0K₂O 2.0-5.0 A1₂O₃ 0.5-5.0 Nucleating agent 2.0-5.0 Me(II)O   0-3.0with Me(II)O being selected from at least one of CaO, BaO, MgO and SrO,and comprises less than 0.1 wt. % of ZnO.

It is preferred that the glass ceramic is essentially free of ZnO.

It is surprising that even without presence of ZnO the glass ceramicaccording to the invention fulfils the mentioned multiple requirements.This was possible by the selection of the other components and theiramounts and preferably by the ratios of some of these components to eachother.

Preferred glass ceramics are those which comprise lithium metasilicateas main crystalline phase. Such glass ceramics are also referred to inthe following as lithium metasilicate glass ceramics.

It has also been found out to be beneficial if the glass ceramiccomprises 0 to 2.0 and preferably 0 to 1.5 wt. % of Me(II)O. Me(II)O isin particular selected from at least one of Cao and MgO. Particularlypreferred glass ceramics comprise 0.1 to 1.0 wt. % of MgO.

The nucleating agent is preferably at least one of P₂O₅ and compounds ofthe elements Pt, Ag, Cu and W. It serves to induce formation of lithiummetasilicate crystals and is preferably P₂O₅.

Further, it was shown that a specific molar ratio of SiO₂ to Li₂O servesto ensure that upon the necessary heat treatment of a correspondingstarting glass mainly lithium metasilicate and lithium disilicate,respectively, is produced. This is of particular importance. While alithium metasilicate glass ceramic essentially free of lithiumdisilicate results in particular in an excellent machinability, alithium disilicate restoration essentially free of the easilydissolvable lithium metasilicate has a very good chemical stability.

Thus, it was found preferable that the molar ratio of SiO₂:Li₂O is atleast 2.2:1, preferably at least 2.3:1, and most preferred in the rangeof 2.3:1 to 2.5:1.

Moreover, investigations revealed that the molar ratio of Al₂O₃:K₂O isof significance for obtaining the desired tranclucence and thepredominant crystallization of lithium metasilicate.

It is preferred that the molar ratio of Al₂O₃:K₂O is in the range of1:0.5 to 1:2.0 and preferably is from 1:1 to 1:2.0.

There also exist preferred ranges for the amounts of components of theglass ceramic according to the invention. These can be usedindependently from each other.

It is preferred that the glass ceramic comprises 2.5 to 5.0 wt. % ofAl₂O₃.

It is also preferred that the glass ceramic comprises 70.0 to 73.0 wt. %of SiO₂.

It is also preferred that the glass ceramic comprises 0 to 4.0,preferably 0.1 to 4.0, more preferably 1.0 to 4.0 and most preferred 1.5to 3.0 wt. % of ZrO₂. If the emphasis is on the achieving of a highstrength of the final lithium disilicate ceramic, then 0 to 2.0 wt. % toof ZrO₂ are advantageous.

It is further preferred that the glass ceramic comprises at least one ofthe following components in an amount of:

Component wt. -% Li₂O 14.0-16.0 K₂O 3.0-4.5 coloring and fluorescentmetal oxides 0-7.5, preferably 0.5-3.5.

The metal of the coloring and fluorescent metal oxides is preferablyselected from group f-elements and in particular from the group of Ta,Tb, Y, La, Er, Pr, Ce, Ti, V, Fe and Mn. The colouring or fluorescentoxides ensure that the colour of the final dental product matches thatof the natural tooth material of the patient in question.

Further, the glass ceramic may comprise as additional component Na₂O inan amount of 0 to 2.0 wt.-%.

Additional components to enhance the technical processability of theglass may also be present. Such additional components may therefore bein particular compounds such as B₂O₃ and F which in general amount to 0to 5.0% by weight.

Generally the amount of lithium metasilicate is 20 to 80 vol.-%. It hassurprisingly been shown that a specific volume portion of lithiummetasilicate should be present to achieve excellent processingproperties. Thus, it is further preferred that the lithium metasilicatecrystalline phase forms 20 to 50 vol % and in particular 30 to 40 vol %of the lithium silicate glass ceramic. Such a part of the volume leadsto the crystals being present rather remote from each other and henceavoids a too high strength of the glass ceramic.

If the emphasis is on the achieving of a high strength of the lithiumdisilicate ceramic, then the lithium metasilicate phase preferably formsmore than 50 and up to 80 vol. % of the lithium silicate glass ceramic.

The lithium metasilicate crystals are preferably of lamellar or plateletform. This leads to a very good machinability of the lithiummetasilicate glass ceramic without use of high energy and withoutuncontrolled breaking. The latter aspect of uncontrolled breaking is forexample known from glasses which are generally unsuitable for machining.It is assumed that the preferred morphology of the lithium metasilicatecrystals is also responsible for the surprisingly high edge strength ofproducts, e.g. complicated dental restorations, can be made from thelithium metasilicate glass ceramic according to the invention.

The lithium silicate glass ceramic according to the invention preferablyis in the form of a blank. The blank usually takes the form of a smallcylinder or a rectangular block. The exact form depends on the specificapparatus used for the desired computer-aided machining of the blank.

After the machining, the lithium silicate glass ceramic has preferablythe shape of a dental restoration, such as an inlay, an onlay, a bridge,an abutment, a facing, a veneer, a facet, a crown, a partial crown, aframework or a coping.

A lithium silicate glass ceramic according to the invention whichcomprises lithium disilicate as main crystalline phase is a furtherpreferred embodiment of the invention. It is preferred that this lithiumdisilicate glass ceramic is formed in a process wherein the lithiummetasilicate of a glass ceramic according to the invention is convertedto lithium disilicate crystals.

A dental product made from lithium disilicate glass ceramic according tothe invention is a further preferred embodiment of the invention. It ispreferred that such product is formed in a process wherein the lithiummetasilicate of a glass ceramic according to the invention is convertedto lithium disilicate crystals.

The lithium metasilicate glass ceramic according to the invention ispreferably prepared by a process which comprises

-   -   (a) producing a starting glass containing the components of the        glass ceramic,    -   (b) subjecting the starting glass to a first heat treatment at a        first temperature to give a glass product which contains nuclei        suitable for forming lithium metasilicate crystals,    -   (c) subjecting the glass product to a second heat treatment at a        second temperature which is higher than the first temperature to        obtain the lithium silicate glass ceramic with lithium        metasilicate as the main crystalline phase.

In step (a), usually a melt of a starting glass is produced whichcontains the components of the glass ceramic. For this purpose acorresponding mixture of suitable starting materials, such ascarbonates, oxides, and phosphates, is prepared and heated totemperatures of, in particular 1300 to 1600° C., for 2 to 10 hours. Inorder to obtain a particularly high degree of homogeneity, the glassmelt obtained may be poured into water to form glass granules and theglass granules obtained are melted again.

It is further preferred that the melt of the starting glass is cooled,such as to room temperature, before subjecting it to step (b). The meltof the starting glass is also usually poured into a mould to form astarting glass blank.

In some cases it is convenient to control a cooling procedure such thatit not only relaxes the glass, but also effects the first heat treatmentof step (b).

In step (b) the starting glass is subjected to a first heat treatment ata first temperature to cause formation of nuclei for lithiummetasilicate crystals. Preferably, this first heat treatment comprisesheating the starting glass to a temperature of 500 to 600° C. for aperiod of about 10 minutes to 3 hours. This results in formation of agreat number of nuclei that ensure a very satisfactory crystal growth.It also ensures that in the further processing after step (c) to give alithium disilicate glass ceramic a very homogeneous lithium disilicatestructure can be obtained.

It is also advantageous that the second heat treatment in step (c)comprises heating the glass product to a second temperature of 570° to750° C., preferably 570 to 670° C., and more preferably to about 650° C.

It has further surprisingly been shown that relatively high temperatureslead to high amounts of lithium metasilicate which in turn lead to ahigh amount of lithium disilicate in the third heat treatment. Such highamounts of lithium disilicate impart a high strength to the ceramic.Thus, if the emphasis is on the achieving a high strength final product,then it is advantageous to carry out the second heat treatment at 680°to 720° C., and preferably 690° to 710° C. and more preferably about700° C.

Depending on the specific composition of a selected starting glass, itis possible for the skilled person by means of differential scanningcalorimetry (DSC) and x-ray diffraction analyses to determine suitableconditions in steps (b) and (c) to result in glass ceramics having thedesired morphology and size of the crystals of lithium metasilicate.Moreover, these analyses allow also the identification of conditionsavoiding or limiting the formation of undesirable other crystallinephases, such as of the high-strength lithium disilicate, or ofcristobalite and lithium phosphate.

Usually, the starting glass of step (a), the glass product of step (b),or preferably the lithium metasilicate glass ceramic of step (c) isshaped to a desired geometry by machining or by hot pressing. Themachining is in particular performed by grinding, trimming or millingand preferably controlled by a computer using CAD/CAM-based millingdevices. This allows a so-called chair-side treatment of the patient bythe dentist.

It is a particular advantage of the lithium metasilicate glass ceramicaccording to the invention that it can be shaped by machining withoutthe undue wear of the tools observed with the tough and high-strengthprior art materials. This is in particular shown by the easy possibilityto polish and trim the glass ceramics according to the invention. Suchpolishing and trimming processes therefore require less energy and lesstime to prepare an acceptable product having the form of even verycomplicated dental restorations.

Further, the lithium metasilicate glass ceramic according to theinvention can advantageously be processed to a lithium disilicate glassceramic of high strength, which usually has a content of 50 to 85 vol. %and preferably 65 to 80 vol. % of crystalline lithium disilicate phase.

This is preferably effected by a process wherein the prepared lithiummetasilicate glass ceramic of step (c) is subjected to a third heattreatment at a third temperature of 830 to 880° C. for a period of 10 to60 minutes. This heat treatment can also be effected when hot-pressingthe lithium metasilicate glass ceramic to achieve a shaping.

Thus, the lithium metasilicate glass ceramic can be further processed tothe lithium dilsilicate glass ceramic of desired shape e.g. by both (i)CAD/CAM and a heat treatment or (ii) a hot-pressing. This is veryadvantageous for the user.

It is also possible to use for these purposes a corresponding lithiumsilicate glass which comprises nuclei suitable for formation of lithiummetasilicate crystals. This glass is a precursor of the lithiummetasilicate glass ceramic and the lithium disilicate glass ceramic ofthe invention. The invention is also directed to such a glass. It isobtainable by the above process in step (b). This lithium silicate glassaccording to the invention comprises the following components:

Component wt. -% SiO₂ 64.0 to 75.0, in particular 64.0-73.0 Li₂O13.0-17.0 K₂O 2.0-5.0 A1₂O₃ 0.5-5.0 Nucleating agent 2.0-5.0 Me(II)O  0-3.0

-   -   with Me(II)O being selected from at least one of CaO, BaO, MgO        and SrO, and which comprises less than 0.1 wt. % of ZnO, and        comprises nuclei suitable for formation of lithium metasilicate        crystals.

For manufacturing a dental restoration by the hot pressing technique, itis preferred to use a lithium silicate glass ingot according to theinvention having nuclei for lithium metasilicate. This ingot is heatedto about 700 to 1200° C. to convert it into a viscous state. The heattreatment can be conducted in a special furnace (EP 500®, EP 600®,Ivoclar Vivadent AG). The ingot is embedded in a special investmentmaterial. During the heat treatment the ingot will be crystallized. Themain crystal phase is then lithium disilicate. The viscous glass ceramicflows under a pressure of 1 to 4 MPa into the cavity of the investmentmaterial to obtain the desired shape of the dental restoration. Aftercooling the investment mould to room temperature the lithium disilicaterestoration can be divested by sand blasting. The restoration can befurther coated with a glass or a glass ceramic by sintering or a hotpressing technique to obtain the finalized dental restoration withnatural aesthetics.

The same hot-pressing technique can be applied to the lithiummetasilicate glass ceramic according to the invention which will beconverted to lithium disilicate glass ceramic.

A preferred method for converting the lithium metasilicate glass ceramicaccording to the invention to a lithium disilicate glass ceramic dentalrestoration by the CAD/CAM technique uses lithium metasilicate glassceramic blanks, e.g. blocks, having a strength of about 80 to 150 Mpa.These can be easily machined in a CAM unit like Cerec 2® or Cerec 3®(Sirona, Germany). Larger milling machines such as DCS Precimill® (DCS,Switzerland) are also suitable. The block is therefore positioned in thegrinding chamber by a fixed or integrated holder. The CAD constructionof the dental restoration is done by a scanning process or an opticalcamera in combination with a software tool. The milling process needsfor one unit about 10 to 15 minutes. Copy milling units such as Celay®(Celay, Switzerland) are also suitable for machining the blocks. First,a 1:1 copy of the desired restoration is fabricated in hard wax. The waxmodel is then mechanically scanned and 1:1 mechanically transmitted tothe grinding tool. The grinding process is therefore not controlled by acomputer. The milled dental restoration has to be subjected to the thirdheat treatment to obtain the desired lithium disilicate glass ceramicwith high strength and tooth-like color. The product can be furthercoated with a glass or a glass ceramic by sintering or hot pressingtechnique to obtain the final dental restoration with naturalaesthetics.

The lithium metasilicate glass ceramic according to the invention canalso be used for coating a dental restoration. The coating is preferablyeffected by hot-pressing the lithium metasilicate glass ceramic onto therestoration.

It was surprisingly found that the easily machinable lithiummetasilicate glass ceramic according to the invention can be convertedby a further heat treatment into a lithium disilicate glass ceramicproduct having also excellent optical properties. The conversion to alithium disilicate glass ceramic is associated with a very small linearshrinkage of only about 0.2 to 0.3%, which is almost negligible incomparison to a linear shrinkage of up to 30% when sintering ceramics.The obtained lithium disilicate glass ceramic has not only excellentmechanical properties, such as high strength, but also displays otherproperties required for a material for dental restorations. It isemphasized that these properties are achieved without the need for ZnOas a component which may be detrimental for specific restorations inview of its strong opalescent effect.

Thus, a product is finally obtained which has all the beneficialmechanical, optical and stability properties making lithium disilicateceramics attractive for use as dental restorative materials. However,these properties are achieved without the disadvantages of theconventional materials when shaped by using a CAD/CAM based process, inparticular the undue wear of the milling and trimming tools.

The invention is explained in more detail below on the basis ofExamples.

EXAMPLES Examples 1 to 8

A total of 8 different lithium metasilicate glass ceramics according tothe invention with the chemical compositions given in Table I wereprepared using the indicated second heat treatment. The obtained glassceramics were then converted to the corresponding lithium disilicateglass ceramics using the indicated third heat treatment.

Firstly, samples of the corresponding starting glasses were melted in aplatinum-rhodium crucible at a temperature of 1450° C. and for a periodof 40 minutes. The glass melt was poured into water and the obtainedgranules were, after drying, again melted at 1500° C. The glass meltsobtained were then poured into graphite moulds to give blocks. Afterrelaxation of the glass blocks at 500 to 600° C. for 10 minutes to 3hours, they were subjected to the given second heat treatment. Beforeeffecting the third heat treatment, the blocks were checked for theirmachinability by milling in a CAD-CAM milling machine (i.e. CEREC 3®).Finally, the indicated third heat treatment was conducted. The crystalphases present after the second and third heat treatment were identifiedby XRD techniques and are given in table I.

Further, the opalescence of the products was visually assessed and thecontrast value CR was determined according to BS 5612 (British Standard)using a spectral colorimeter (Minolta CM-3700d). The chemical stabilityin acetic acid was determined as well as the stability in artificialsaliva. The corresponding data are to be found in the following Table IIand show in particular the surprising combination of a lack ofopalescence together with a high translucence and stability. Thecomposition of the artificial saliva is given in table III.

The data obtained show that the lithium metasilicate glass ceramicsaccording to the invention combine a very good machinability and highedge strength with the easy possibility to convert them by a simple heattreatment into lithium disilicate glass ceramics which have a very highbending strength as well as an excellent chemical durability and goodtranslucence, all of which being properties which make them veryattractive as materials useful for the manufacture of dentalrestorations.

Examples 9 to 12

Four glass ceramics according to the invention were prepared inanalogous manner as examples 1 to 8. However, the heat treatment schemewas different. In addition each material was subjected to the schemesreferred to as “Cycle A” and “Cycle B” which differ in the temperatureused for the crystallization of lithium metasilicate, namely 650° and700° C., respectively.

Details as to the materials prepared and tested as well as theirproperties are given in the table IV. It is apparent that the “Cycle B”treatment using a temperature of 700° C. for the crystallization oflithium metasilicate leads to lithium disilicate glass ceramics havingexcellent strengths.

TABLE I Example 1 2 3 4 5 6 7 8 Molar ratio SiO2:Li2O 2.39:1 2.39:12.4:1 2.39:1 2.39:1 2.39:1 2.39:1 2.39:1 Al2O3:K2O   1:1.0   1:1.0  1:1.2    1:1.20    1:1.35    1:1.50    1:1.70    1:1.30 wt.-% (Mol %)wt.-% (Mol %) wt.-% (Mol %) wt.-% (Mol %) wt.-% (Mol %) wt.-% (Mol %)wt.-% (Mol %) wt.-% (Mol %) SiO2 72.21 (66.12) 70.64 (65.62) 70.52(65.52) 70.78 (65.57) 70.78 (65.56) 70.78 (65.56) 70.78 (65.55) 70.78(65.29) K2O 3.16 (1.85) 3.09 (1.83) 3.81 (2.26) 3.76 (2.22) 3.96 (2.34)4.16 (2.46) 4.36 (2.58) 3.36 (1.98) Li2O 14.99 (27.60) 14.68 (27.43)14.64 (27.35)  14.7 (27.38)  14.7 (27.38)  14.7 (27.37)  14.7 (27.37) 14.7 (27.26) Al2O3 3.45 (1.86) 3.38 (1.85) 3.35 (1.83) 3.38 (1.85) 3.18(1.74) 2.98 (1.63) 2.78 (1.52) 2.78 (1.51) P2O5 3.28 (1.27) 3.21 (1.26) 3.2 (1.26) 3.21 (1.26) 3.21 (1.26) 3.21 (1.26) 3.21 (1.26) 3.21 (1.25)ZrO2 2.91 (1.30) 3.00 (1.36)  2.5 (1.13)  1.8 (0.81)  1.8 (0.81) 1.80(0.81)  1.8 (0.81)  1.8 (0.81) CeO2 1.88 (0.61) 1.86 (0.60) 2.00 (0.65)2.00 (0.65) 2.00 (0.65) 2.00 (0.65) 2.00 (0.65) V2O5 0.12 (0.04) 0.12(0.04)  0.07 ((0.02)  0.07 ((0.02)  0.07 ((0.02) 0.07 (0.02) 0.07 (0.02)MnO2 0.03 (0.02) 0.03 (0.02) 0.03 (0.02) 0.03 (0.02) 0.03 (0.02) Er2O3 0.12 (0.017)  0.12 (0.017) 0.12 (0.17)  0.12 (0.017)  0.12 (0.017) MgO0.15 (0.21) 0.15 (0.21) 0.15 (0.21) 0.15 (0.21) 0.15 (0.21) CaO 1.00(0.99) Crystalline phases after: Second heat Li2SiO3 Li2SiO3 Li2SiO3Li2SiO3 Li2SiO3 Li2SiO3 Li2SiO3 treatment: Li2Si2O5 * Li2Si2O5 *Li2Si2O5 20′/650° C. Third heat Li2Si2O5 Li2Si2O5 Li2Si2O5 Li2Si2O5Li2Si2O5 Li2Si2O5 Li2Si2O5 Li2Si2O5 treatment: Li3PO4* Li3PO4* Li3PO4*Li3PO4* Li3PO4* Li3PO4* Li3PO4* Li3PO4* 10//850° C.

TABLE II Example 1 2 3 4 6 CR-Value BS- 40.4 37.0 50.0 59.3 58.8 5612(1978) Opalescence No No No No No Chemical stability 9 18 48 3 9 inAcetic acid (24 h/80° C., mass loss in μg/cm²) Chemical stability 13 1728 27 17 in Saliva (7 d/60° C., mass loss in μg/cm²)

TABLE III Composition of artificial saliva Component Amount in mg in atotal of 500 ml H₂O NaC1 125.64 KC1 963.9 NH₄Cl 178.0 CaCl₂•2H₂O 227.8KSCN 189.2 CO (NH₂)₂ 200.0 Na₂SO₄•10H₂O 336.2 NaHCO₃ 630.8 KH₂PO₄ 654.5

TABLE IV Example 9 10 11 12 SiO₂ 74.37  72.89  72.21  71.40  K₂O 3.263.18 3.16 3.13 Li₂O 15.44  15.13  14.99  14.79  A1₂O₃ 3.55 3.48 3.453.41 P₂O₅ 3.38 3.31 3.28 3.22 ZrO₂ 0.00 2.01 2.91 4.05 All values abovein wt.-% SiO₂:Li₂O 2.39 2.40 2.39 2.40 (Mol.-ratio %) Cycle A: (1) 500°C./10 min + (2) 650° C./20 min + (3) 850° C./10 min *) Biaxial 786 +/−92  515 +/− 54 522 +/− 82 479 +/− 36  Flexural Strength/MPa ContrastRatio 0.80 0.56 0.43 0.36 Cycle B: (1) 500° C./10 min + (2) 700° C./20min + (3) 850° C./10 min *) Biaxial 828 +/− 104 659 +/− 75 608 +/− 90694 +/− 113 Flexural Strength/MPa Contrast Ratio 0.83 0.63 0.53 0.41 *)(1) Nucleation in the glass (2) Crystallization of Li-Metasilicate (3)Crystallization of Li-Disilicate from Li-Metasilicate

What is claimed:
 1. A lithium silicate glass ceramic for dentalrestorations which comprises a main crystalline phase comprising lithiummetasilicate, wherein the lithium metasilicate crystals compriselamellar or platelet crystals.
 2. The lithium silicate glass ceramic ofclaim 1, wherein the lithium metasilicate forms more than 50 and up to80 vol. % of the lithium silicate glass ceramic.
 3. The lithium silicateglass ceramic of claim 1, wherein the glass ceramic is in form of ablank or a dental restoration.
 4. The lithium silicate glass ceramic ofclaim 3, wherein the blank is shaped into a dental restoration bymachining or pressing.
 5. The lithium silicate glass ceramic of claim 4,wherein machining comprises grinding, trimming or milling.
 6. Thelithium silicate glass ceramic of claim 4, wherein the dentalrestoration is an inlay, an onlay, a bridge, an abutment, a facing, aveneer, a facet, a crown, a partial crown, a framework or a coping. 7.The lithium silicate glass ceramic of claim 1, comprising SiO₂, Li₂O anda nucleating agent.
 8. A lithium silicate glass ceramic material formedby: a) heating a starting glass material which is essentially free ofZnO and comprises SiO₂, LiO₂, K₂O, Al₂O₃, nucleating agent, andoptionally Me(II)O, wherein Me is CaO, BaO, MgO, SrO or combinationsthereon, at a first temperature such that nuclei suitable to formlithium metasilicate crystals are formed; and b) heating the materialfrom a) at a second temperature which is higher than the firsttemperature to produce lithium silicate glass ceramic which has lithiummetasilicate as the main crystalline phase, and wherein the crystals areof lamellar or platelet form, wherein the material formed after step b)is convertible to a lithium silicate glass ceramic having disilicate asthe main crystalline phase by heating to a third temperature which ishigher than the second temperature.